Radiation chemotherapy combination is a major remedy in treating a broad spectrum of solid tumors. The clinical efficacy of chemo-radiation combination is enhanced by a strong synergy effect between radiation and DNA alkylating drugs. To date, the mechanism(s) mediating the synergistic effect remains elusive. The genetic determinants of cellular sensitivity/resistance against chemo-radiation treatment remain undefined. This application is aimed at addressing these important questions via genetic approaches. Our long term objective is to reveal genes that play a crucial role in cellular resistance against chemo-radiation combination and the molecular basis of synergy between radiation and chemotherapy. To identify these genes in an unbiased manner, we have established high throughput platforms for genome-wide RNAi screen. An initial pilot screen has validated technical platform and acquired proof of principle results. The immediate goals of this R21 application are reflected by two Specific Aims, 1. To conduct multiple genome-wide primary screens for candidate genes critical for cellular survival against chemo-radiation treatment; 2. To refine the primary hits in a secondary screen to functionally validate genes acting as synergy factors and genes prevalent in cellular resistant to chemo-radiation treatment. Unraveling genetic determinants of chemo-radiation response provides potential biomarkers for prognosis and treatment rationalization. Elucidating the biological underlining for synergy helps to derive novel therapeutic combinations. We expect this high risk/high reward project to yield clinically relevant as well as mechanistically informative candidates that could lead to further in- depth investigations to improve the chemo-radiation therapeutic regimen.